Ten years ago this week a UN peacekeeping mission was sent to East Timor, bringing an end to a wave of violence by Indonesian-sponsored militia gangs and paving the way for the country's transition to independence.

The debate about "humanitarian interventions" has been overshadowed by the subsequent western military interventions in Iraq and Afghanistan. Although neither of these can be classified as humanitarian in the generally accepted sense of the term, the marshalling of human rights arguments in defence of them has created a scepticism towards the notion that the use of external military force can ever be justified on humanitarian grounds.

In calling for the deployment of "an effective UN presence" in East Timor in September 1999, Amnesty came closer than it has ever done to supporting military intervention. Although I still think we were right to do so, it is worth revisiting these discussions 10 years on.

Two of the most basic rules of international law are that states should respect one another's sovereignty and find peaceful ways of settling their disputes. The UN Charter only permits a resort to force on two grounds: self-defence and where such action has been authorised by the UN security council. These latter grounds were used increasingly during the 1990s in response to humanitarian crises in places such as Rwanda and Bosnia-Herzegovina.

The weaknesses of some of these UN missions and the fact that the five permanent members of the security council all had a veto on such operations has led some legal scholars to argue for the development of a new norm in international relations that would legitimise interventions on humanitarian grounds. This argument is controversial, not least because while it is easy to think of a hypothetical situation where such an intervention may be justified without a UN mandate, it is much harder to find real-life examples of where this was actually a practical possibility.

East Timor is probably the best case that can be made. The intervention was authorised but it took several precious days of shuttle diplomacy to obtain the consent of the Indonesian government; the Chinese had indicated they would veto a deployment without this consent. A well-equipped force of 11,000 troops, led by Australia, was assembled and deployed within three weeks from the start of the crisis and it is doubtful this could have been done any faster even without the diplomatic wrangling. I do remember, however, a serious internal conversation about whether Amnesty should press for intervention without UN authorisation.

I was in Timor-Leste earlier this year and the country still faces many of the same problems that have confronted it since independence. Security sector reform is lagging, the justice system is weak. The government shows signs of intolerance towards dissenting voices, and it has not got a grip on corruption. President Ramos-Horta was shot and wounded in February 2008 and his government remains heavily dependent on external support. The story is much the same in other post-conflict countries, such as Liberia, Sierra Leone, Haiti, Kosovo and Bosnia, which remain to all intents and purposes international protectorates.

Yet the world is a more peaceful place than it was 10 years ago – and while undoubtedly that is due to a variety of external factors, peacekeeping missions have helped to dampen down some conflicts. We have also learnt a lot of lessons – partly through our own failures – and programmes such as Demobilisation, Disarmament and Reintegration (DDR) aimed at child soldiers or land dispute conflict resolution projects have helped to heal some of the wounds in broken societies. Civilian military co-operation has also improved on most missions, based on mutual respect for our different mandates; aid workers don't want to be soldiers and soldiers are gradually learning that they don't know how to deliver aid.

Of course the exceptions to all of this are Afghanistan and Iraq where attempts have been made to integrate aid into counter-insurgency campaigns. The failings of this have been so well-documented elsewhere as to not need repeating, although perhaps if politicians had studied some of the actual humanitarian interventions that have taken place over the last 10 years some of these mistakes could have been avoided.

First of all, humanitarian interventions are massively expensive. It took 60,000 international troops to secure tiny and peaceful post-Dayton Bosnia-Herzegovina and a similar-sized army of international administrators and aid and reconstruction workers. Did President Bush ever really think that 4,500 soldiers could do the job in Afghanistan?

Second, the international governance structures we create are usually inefficient and often dysfunctional. The international presence is almost always resented by the local population, partly due to the "lavish lifestyles" of expatriate staff and their overwhelming ignorance about the country in which they are working. Virtually every mistake that was made in Iraq had been made somewhere else before.

Third, external interventions have a massive distorting effect on both the economic and political life of the country concerned. A huge injection of resources almost always damages local markets and feeds official corruption. Imposing governance and assistance mechanisms from the outside will always weaken local accountability, and we have yet to design effective exit strategies from most of the countries concerned. The international courts and tribunals have a mixed record and it has been repeatedly shown that equating the organisation of elections with the development of a functioning democracy has been hopelessly naive.

None of this is to argue against the notion that there will be circumstances in which massive and systematic violations of human rights justify an external military intervention, but it is to stress why it should be an absolute last resort. As the challenges of such interventions have become increasingly apparent so has the realisation about how overstretched are the international resources devoted to them – which mainly accounts for the weakness of the responses in places like the Democratic Republic of Congo and Darfur.

These are logistical rather than ideological arguments against ill-thought-out interventions. The growing influence of international human rights law has undermined the Westphalian notion that what a government does to its own people is a prerogative of national sovereignty, but nor should it be forgotten that the principle of self-determination is written into not just the UN Charter, but as the first article in the twin covenants that arose from the Universal Declaration of Human Rights.